Some communication standards for electronic devices to communicate with each other may allow one electronic device to supply electric power to another electronic device through a communication cable. The typical communication standard is USB (Universal Serial Bus). For example, a portable electronic device, which is difficult to receive an electric power from a commercial alternating current power supply, may be activated to perform a normal operation or charge a built-in battery by being powered through a communication cable.
Moreover, the specification related to the electric power supplied through a communication cable differs depending on the communication standard. For example, in the case of USB, in each of the USB 1.1 standard and the USB 2.0 standard, the output voltage is 5 V and the output current is 0.5 A. In contrast, in the USB 3.0 standard, the output voltage is also 5 V but the output current is 0.9 A.
Note that examples of the technique for determining the communication standard, which a connected electronic device supports, include an electronic device configured to determine the communication standard, which a connected host device supports, based on the potential of a communication ground terminal in the USB 3.0 standard among the terminals of a connector supporting both the USB 2.0 standard and the USB 3.0 standard.
Japanese Laid-open Patent Publication No. 2011-203781
As described above, in an electronic device powered through a communication cable, the electric power to be supplied is preferably larger. For example, in electronic devices, as the electric power to be supplied increases, the charging of a built-in battery may be completed in a shorter time.
As with the USB standard, in the case of a communication standard, in which the specification of the power supply differs depending on the generation of the communication standard and the physical backward compatibility of a communication cable is ensured, one of connected electronic devices may support an old communication standard while the other one may support a new communication standard. In this case, although the other connected electronic device has a larger power supply capability specified by the new communication standard, the electronic device supporting the old communication standard sets the setting related to the supplied electric power so as to consume only a small electric power specified by the old communication standard. Because the electronic device supporting the old communication standard is unable to determine whether the other connected electronic device supports the new communication standard.